Heat exchanger tube with enhanced heat transfer co-efficient and related method

ABSTRACT

A method of enhancing the heat transfer coefficient of a tube flowing a fluid in the tube in heat exchange relation with a second fluid external to the tube includes applying metal particles to an interior surface of the tube to thereby create an increased wetted area for interaction with fluid flowing through the tube; wherein the particles are applied by: providing an applicator head including an electric arc and at least one metal wire; displacing the head along the interior of the tube; feeding the wire into the electric arc as the head is displaced along the interior of the tube; melting the wire to form molten metal particles; and applying the molten metal particles about and along the length of the interior wall surface of the tube as the head is displaced along the interior of the tube.

BACKGROUND

The present invention relates to heat exchangers and particularly to aircooled heat exchangers having an increased heat transfer co-efficientbetween the fluid flowing within the tube and the tube itself.

Heat exchangers providing heat exchange between a fluid within a seriesof tubes and cooling air flowing about the tubes are well known.Enhancements to these heat exchangers have taken the form of a pluralityof fins applied externally about the tubes enhancing the heat exchangebetween the cooling air flowing about the tubes and fins and the fluidflowing within the tubes. Various methods for increasing the exchangesurface and heat transfer co-efficient are well known in otherenvironments, such as the use of brazed micro turbulators on internalsurfaces of gas turbine parts, and internal tube dimpling. See, forexample, U.S. Pat. Nos. 6,598,781 and 6,644,921. However, theseprocesses have not been applied to air cooled heat exchangers, and donot address the enhancement of heat transfer between the fluid within atube and the tube itself. Accordingly, there remains a need forincreased heat exchange between the fluid inside a tube and the tubewall in an air cooled heat exchanger.

BRIEF SUMMARY

This invention increases the wetted area inside a heat exchanger by oneof two methods.

In one exemplary embodiment, random or patterned microturbulators in theform of metal particulates are bonded to the internal surface of thetube by a method described further herein. The microturbulated surfacesallow fluid to interact with all portions of the increased wetted area,and also create fluid-transitional or turbulent flows. The preferredmethod for applying particles to the interior surface of a heatexchanger tube is a wire spray process that ablates two fed wires heldat opposite electrical polarity. A single wire spray process is alsocontemplated. The resultant molten metal particles fuse to the interiorof the tube, thus creating a random or patterned array ofmicro-turbulator particles onto the tube interior surface.

A second method uses a movable/retractable welding tip to apply weldingmaterial to the interior surface of the tube in any desired pattern thatpromotes and enhances heat transfer. For example, weld material may beapplied so as to form a continuous spiral along the length of the tubeinterior. Alternatively, material may be added in discrete amounts toform patterned roughness on the interior surface of the tube.

Each of the methods briefly described above results in bonded internaltube surface microturbulations that enhance thermal performance.

Accordingly, in one aspect, the present invention relates to a method ofenhancing the heat transfer coefficient of a tube flowing a fluid in thetube in heat exchange relation with a second fluid external to the tubecomprising applying metal particles to an interior surface of the tubeto thereby create an increased wetted area for interaction with fluidflowing through the tube. The particles may be applied by: providing anapplicator head including an electric arc and at least one metal wire;displacing the head along the interior of the tube; feeding the wireinto the electric arc as the head is displaced along the interior of thetube; melting the wire to form molten metal particles; and applying themolten metal particles about and along the length of the interior wallsurface of the tube as the head is displaced along the interior of thetube.

In another aspect, the invention relates to a method of enhancing heattransfer characteristics of a heat exchanger tube adapted to carry afluid there through, the method comprising: locating a MIG welding torchnozzle within the heat exchanger tube; moving the torch nozzle axiallyrelative to the tube while applying welding material to and along theinterior surface of the tube to thereby enhance heat transfer betweenthe tube and the fluid flowing through the tube.

In another aspect, the invention relates to a heat exchanger tubecomprising a hollow tube having an interior surface substantiallycovered with metal particles for enhancing heat exchanger properties ofthe tube.

The invention will now be described in connection with the drawingsidentified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art heat exchanger;

FIG. 2 is a schematic illustration of a tube with fins forming part of aprior art heat exchanger of FIG. 1;

FIG. 3 is a schematic illustration of an electric dual wire spray forspraying particulates onto the interior surfaces of the heat exchangertube in accordance with another exemplary embodiment;

FIG. 4 is a simplified side elevation of conventional MIG weldingapparatus that may be used in another exemplary embodiment of theinvention; and

FIG. 5 is a perspective cut-away of a tube formed with an internalspiral groove, not necessarily to scale, using the apparatus of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, particularly to FIG. 1, there is illustrateda conventional heat exchanger generally designated 10. Heat exchanger 10is comprised of a plurality of interconnected tubes 12 forcarrying/circulating a fluid to be cooled. The hot fluid is typicallyconveyed back and forth in opposite directions (or in one way only) intubes arranged in a large grid-like pattern. In the illustrated form,the tubes 12 extend from a hot fluid inlet 14, back and forth in thegrid pattern and terminate at an outlet port 16. It will be understoodthat the tubes can be arranged in many different configurations, e.g.,one above the other, in layers offset one above the other or in anyother well-known and suitable configuration. It will be appreciatedthat, in use, the tubes 12 are in heat exchange relation with a coolingfluid e.g., air flowing across and through the grid-like pattern. Itwill also be appreciated that the tubes may carry a fluid to be heatedby flowing a heated fluid across the tubes.

To facilitate the heat transfer, and using as an example heat exchangebetween tubes carrying a hot fluid and air passing over and about thetubes, a fan 18 with fan blades 20 is disposed, for example, below thetubes 12 for driving air through and across the grid. Thus, the air andthe tubes 12 are in heat exchange relation one with the other such thatthe heated fluid passing through the tubes 12 is cooled and exits theheat exchanger at outlet port 16 at a lower temperature than fluid atthe inlet 14. This invention also contemplates situations where only thelatent heat is involved, such that the fluid will have energy removed,but will not actually be cooled.

An enlarged schematic illustration of a finned tube 12 is shown in FIG.2. Thus, the tubes 12 in the heat exchanger may carry fins 22 which areattached to the tubes in a conventional manner. It will be appreciatedthat the fins increase the effective surface area of the interfacebetween the cooling air and hot fluid enabling enhanced thermal coolingof the hot fluid as a result of the finned configuration.

Further enhancement of heat transfer in connection with fins isdescribed in commonly owned co-pending application Ser. No. 11/493,022,filed Jul. 26, 2006.

As used in the description of various embodiments of this invention, theterm “fluid” embraces liquids, gases, steam, two phase mixtures, andmulti-component mixtures. Also, the heat exchanger may be of the typefor condensing or evaporating fluid.

Referring now to FIG. 3, there is illustrated a dual wire spray head inwhich a random or patterned array of microturbulators may be applied andbonded to the interior surface 24 of a heat exchanger tube 26.Specifically, a moveable/retractable dual wire spraying mechanism 28 isutilized to controllably apply the metallic particles to the interiorsurface of the tube (of circular cross section). In this instance, themechanism 28 includes an applicator head 30 having a pair of wires 32,34 which are fed through electrical contact tubes 36, 38, respectively.The head 30 is also constructed to provide primary atomizing gas througha central aperture 40 and secondary atomizing gas through an annularsecondary aperture 42. Any suitable gas may be employed, but air is lesspreferred since it may oxidize the metal particles preventing them frombonding to the tube. It will be appreciated that an electric arc betweenthe wires 32, 34 vaporizes the wires, causing particulates 44 to mixwith the atomizing gas and to be metalurgically bonded to the interiorsurface 24 of the tube 26, without having to apply an adhesive coat orlayer.

Referring now to FIG. 4, a known MIG welding apparatus is illustratedthat is available from Bore Repair Systems, Inc. of Alstead, N.H. Theapparatus is also shown and described in U.S. Pat. No. 6,137,076. Theapparatus includes a weld torch assembly 46 that incorporates a firsthollow arm 48 and a second hollow arm 50 with a clutch control mechanism52 in a housing 54 between the hollow arms. A nozzle 56 at the distalend of arm 50 is arranged to apply weld material to the interior surfaceof a round tube 58, with clamping means 60, adjustable support bracket62 and mounting rod 64 enabling positioning of the nozzle. At the sametime, a threaded guide 66, spindle 68 and clutch control mechanism 52control axial movement of the torch through the tube.

In an exemplary but non-limiting embodiment, the weld torch 46 isrotated as it moves axially through an elongated heat exchange tube 58(FIG. 5) located in the position of tube 58 in FIG. 4.

As the torch rotates and moves axially, the welding ingredients (weldingcurrent, welding wire and welding gas) are supplied through the arms 48and 50 to the nozzle 56, depositing welding material 72 on the innerdiameter surface of the heat exchanger tube 58, as a continuous spiralrib 74 (thus creating a spiral rib/groove configuration) extendingsubstantially the entire length of the tube, as best seen in FIG. 5.Note in FIG. 5, a portion 76 of the tube has yet not been altered, orwill remain in its original configuration and serve, for example, as atube-end connector. Alternatively, weld material may be added to roughenthe interior surface of the tube, the roughness achieved by depositingthe weld material in a random or patterned array.

In each case, the interior surface area of the tube is increased tothereby enhance the heat transfer characteristics of the tube.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of enhancing the heat transfer coefficient of a tube flowinga fluid in the tube in heat exchange relation with a second fluidexternal to the tube comprising applying metal particles to an interiorsurface of the tube to thereby create an increased wetted area forinteraction with fluid flowing through the tube.
 2. The method of claim1 wherein the particles are applied by: providing an applicator headincluding an electric arc and at least one metal wire; displacing thehead along the interior of the tube; feeding the wire into the electricarc as the head is displaced along the interior of the tube; melting thewire to form molten metal particles; and applying the molten metalparticles about and along the length of the interior wall surface of thetube as the head is displaced along the interior of the tube.
 3. Themethod of claim 1 including providing a second wire, melting the secondwire together with the first mentioned wire to form the molten metalparticles, and applying the metal particles of said first and secondwires along the length of the interior wall surface of the tube.
 4. Themethod of claim 1 including mixing the molten particles with atomizedgas to facilitate applying the molten metal particles to the interiorwall surface of the tube.
 5. A heat exchanger tube comprising a hollowtube having an interior surface substantially covered with metalparticles for enhancing heat exchanger properties of the tube, saidmetal particles in molten form when applied to said interior surface andmetalurgically bonded thereto.
 6. A method of enhancing heat transfercharacteristics of a heat exchanger tube adapted to carry a fluid therethrough, the method comprising: locating a MIG welding torch nozzlewithin the heat exchanger tube; moving the torch nozzle axially relativeto the tube while applying welding material to and along the interiorsurface of the tube to thereby enhance heat transfer between the tubeand the fluid flowing through the tube.
 7. The method of claim 6 whereinthe welding material is applied in discrete amounts to increase surfaceroughness of said interior surface.
 8. The method of claim 6 includingrotating the welding torch such that material is added to the interiorsurface of the tube in the form of a continuous spiral rib.